When two dissimilar metals are connected and that junction exposed to an elevated temperature, a thermal electromotive force is generated. This phenomenon is known as the Seebeck effect and is the basis of temperature measurements using thermocouples. When two similar metals are joined, no thermoelectric effect takes place. Thermocouple leads are, therefore, made from the same material from which the thermocouples are made, or are made from metals which minimize the thermoelectric effect. Eventually, in the circuit, however, connections must be made to copper, such as at the binding post of a potentiometer or other measuring device. Unfortunately, these connections form two new junctions which are themselves thermocouples. The error produced by these new thermocouple junctions can be eliminated by placing the copper to thermocouple lead junction in a bath of melting ice, as explained by Mark W. Zemansky in Heat and Thermodynamics. This keeps the two junctions at the same temperature and keeps that temperature constant. This cancels the thermoelectric effect of the new junction. If all additional terminations are copper to copper, no new thermal electromotive error is introduced. The electromotive force generated by the thermocouple is then measured with a potentiometer. The electromotive force generated by the thermocouple when an ice bath is used with the reference junction is represented by the equation: ##EQU1## where E equals electromotive force generated by the thermocouple; t equals temperature in degrees Celsius; A, B, C are constants dependent upon the materials used to fabricate the thermocouples. If the thermocouple to copper lead junctions are at some temperature other than the melting point of ice, then the above equation must be corrected by adding a term dependent upon the temperature of the junction, for example, EQU E=E.sub.c +E.sub.r
where E equals electromotive force generated by thermocouple if an ice junction were used; E.sub.c equals correction factor; E.sub.r equals electromotive force actually measured. Once the electromotive force is determined, it is a simple matter to look up the temperature in a chart of electromotive force versus temperature which have been developed for various thermocouples and published in books such as the CRC Handbook of Chemistry and Physics.
Electronic devices such as a Thermocouple Cold Junction Compensator Model 2B56, manufactured by Analog Devices Incorporated, are available to replace an ice junction as a reference junction. Compensation is then made electrically. An isothermal block such as described in Applicant's co-pending application, Ser. No. 398,537, filed July 15, 1982, is used to terminate the thermocouple leads. An integrated circuit temperature transducer, such as a Model AD-590, manufactured by Analog Devices Incorporated, senses the temperature of the isothermal block. The thermocouple cold junction compensator, which has been pre-programmed for the type of thermocouple being utilized, accepts the signal from the temperature transducer and produces a millivoltage signal output which is representative of the amount of millivoltage produced by that specific type of thermocouple at the temperature of the isothermal block. This analog signal is added to the millivoltage signal produced by the thermocouple to give a corrected or compensated signal.
Two problems present themselves by using this type of system with a plurality of thermocouples. The first problem is the signal produced by a Model 2B56 is an analog signal dependent upon the type of thermocouple which is being compensated. In a complex process, different types of thermocouples may be used simultaneously. A separate Model 2B56 must be used for each type of thermocouple. The second problem also relates to the fact that the Model 2B56 produces an analog signal. Since the Model 2B56 produces an analog signal which must be added to the thermocouple signal, only one thermocouple at a time may be compensated. The Model 2B56 has provisions for switching sequentially or selectively to four individual thermocouples. This precludes simultaneous compensation of a plurality of thermocouples.
The present application overcomes these problems by digitizing the signal from the integrated circuit temperature transducer. The signal from the integrated circuit temperature transducer is converted into a digital signal which is representative of the temperature of the isothermal block. This signal is connected to microprocessors associated with each thermocouple lead by means of a communication bus. The compensation is then made digitally within the microprocessors.